It is known to conduct fracturing or other stimulation procedures in a wellbore by isolating zones of interest, or intervals within a zone, in the wellbore, using packers and the like, and subjecting the isolated zone to treatment fluids, including liquids and gases, at treatment pressures.
In a typical fracturing procedure for a cased wellbore, for example, the casing of the well is perforated to admit oil and/or gas into the wellbore. Fracturing fluid is then pumped into the wellbore and through the perforations into the formation. Such treatment opens and/or enlarges drainage channels in the formation, enhancing the producing ability of the well. For open holes that are not cased, stimulation is carried out directly in the zones or zone intervals.
It is typically desired to stimulate multiple zones in a single stimulation treatment, typically using onsite stimulation fluid pumping equipment. A series of packers in a packer arrangement is inserted into the wellbore, each of the packers located at intervals for isolating one zone from an adjacent zone. It is known to introduce a ball into the wellbore to selectively engage one of the packers in order to block fluid flow therethrough, shift a sleeve to open ports for treatment or stimulation of an isolated zone uphole from a packer. Once the isolated zone has been stimulated, a subsequent ball is dropped to block off a subsequent packer, uphole of the previously blocked packer, for isolation and stimulation thereabove. The process is continued until all the desired zones have been stimulated. Typically, the balls range in diameter from a smallest ball, suitable to block the most downhole packer, to the largest diameter, suitable for blocking the most uphole packer.
At surface, the wellbore is fit with a wellhead including valves and a pipeline connection block, such as a frac head, which provides fluid connections for introducing stimulation fluids, including sand, gels and acid treatments, into the wellbore. Conventionally, operators manually introduce balls to the wellbore through an auxiliary line, coupled through a valve, to the wellhead. The auxiliary line is fit with a valved tee or T-configuration connecting the wellhead to a fluid pumping source and to a ball introduction valve. The operator closes off the valve at the wellhead to the auxiliary line, introduces one ball and blocks the valved T-configuration. The pumping source is pressurized to the auxiliary line and the wellhead valve is opened to introduce the ball. This procedure is repeated manually, one at a time, for each ball.
Other alternative methods and apparatus for the introduction of the balls have included an array of remote valves positioned onto a multi-port connection at the wellhead with a single ball positioned behind each valve. Each valve requires a separate manifold fluid pumper line and precise coordination both to ensure the ball is deployed and to ensure each ball is deployed at the right time in the sequence, throughout the stimulation operation. It is known to feed a plurality of perforation-sealing balls using an automated device as set forth in U.S. Pat. No. 4,132,243 to Kuus. Same-sized balls are used for sealing perforations and are able to be fed one by one from a stack of balls. The apparatus appears limited to same-sized balls and there is no positive identification whether a ball was successfully indexed from the ball stack for injection. In another prior art arrangement, such as that set forth in FIG. 1, a vertically stacked manifold of pre-loaded balls is oriented in a bore above the wellbore of a wellhead and frac head. Each ball is temporarily supported by a rod or finger. Each finger is sequentially actuated to withdraw from the bore when required to release or launch the next largest ball. As the balls are already stacked in the bore, the lowest ball (closest to the wellbore) is necessarily the smallest ball.
Applicant introduced a radial ball injector in U.S. Pat. No. 8,136,585, issued Mar. 20, 2012, the injector having a housing adapted to be supported on the wellhead. Each radial housing has an axial bore and at least one radial ball array having two or more radial bores extending radially away from the axial bore and fluidly connected therewith. The axial bore is aligned with the wellbore. Each radial bore houses a ball cartridge. Each radial bore has an actuator for actuating the ball cartridge. The ball cartridge is movable along the radial bore for extending into and retracting from the axial bore. The ball cartridge receives, stores, and releases balls. More than one radial ball array can be vertically stacked one on top of another to increase the number of balls available for wellbore operations. A radial ball array can be housed in a radial housing. Alternatively, more than two radial ball arrays can be vertically arranged within a radial housing. In each case, the axial bore of each of the radial housing is aligned with one another and with the wellbore.
Regardless of the means for introducing the ball, an operator cannot be assured of the ball's successful introduction to the wellbore, or the timing in which the ball reaches the downhole tool, or the safety and integrity of the ball.
As the majority of the sand-laden fracturing fluids are introduced at the frac head below the ball injectors, there can be an accumulation of sand formed in the bore at the interface of the ball injector and the frac head.
Typically, to reduce the effect of direct impingement of the fracturing fluid on the frac head, frac fluid can be introduced to the frac head from opposing inlets. As these high velocity fluid streams impact one another in the main bore of the frac head, a vortex of fracturing fluid can be created in an upper portion of the bore, which can result in an accumulation of sand or other particulates at the injector, the accumulation potentially impeding ball injection.
Further, as shown in FIG. 5, in undulating horizontal wellbores, sand can accumulate in low spots, potentially impeding the progress of a dropped ball from reaching its destination. Restrictions can also occur at the heel or flow dynamics can trap a ball at flow discontinuities such as the heel or joints as well as the aforementioned surface equipment side ports. These trap zones or accumulations can impede, delay or prevent introduction of the ball into the frac fluid flow or delivery downhole.
As a result, balls can be impeded from entering the wellbore, and for those balls that do successfully enter the wellbore, these can be impeded from reaching their desired and intended location, such as downhole tools.
Further, it is not uncommon for a ball to be damaged or to disintegrate enroute or upon arrival at the downhole tool requiring a replacement ball or one of the same diameter to be launched again. Some apparatus requires depressurization and reloading of balls. This requires time consuming and properly managed procedures to maintain safe control in a hazardous environment and to complete testing and re-pressurization procedures upon reinstallation to the wellhead. The pumping of displacement fluid, such as fracturing fluid, through the ball injector unit can also damage or scar balls, especially if the displacement fluid is sand-laden fracturing fluid. Damaged and scarred packer balls typically fail to isolate the zone requiring an operator to then drop an identical ball down the bore of the injector.
Some injector apparatus, lacking backup balls, can require the entire unit to be removed, the replacement ball dropped, the unit reassembled, and pressure tested. This is extremely inefficient, time consuming, costly and can adversely compromise the treatment. Some methods require several balls to arrive at a multi-port packer basically at the same time. In such a multi-port packer, if some ports close and one of the multi-port remains open, the increased surge of fluid travelling therethrough will erode the orifice very quickly.
Further, other actuators such as a dart and the like are also dropped into a wellbore for actuation of various tools such as packers, landing nipples and circulation subs.
There remains a need for a methodology for delivering dropped balls safely and reliably downhole.